1. Field of the Invention
The present invention relates to a spray head of a rotary type electrostatic spray painting device.
2. Description of the Related Art
In a rotary type electrostatic spray painting device in which paint is fed onto a cup-shaped inner circumferential wall of a spray head while the spray head is rotating, and in which a spray of paint is discharged from a tip of the spray head, the necessary atomization of the paint is greatly influenced by the shape of the tip of the spray head. To improve this atomization of the paint, a rotary type electrostatic spray painting device has been disclosed in which a plurality of paint guide grooves extending to a knife edge-shaped tip of the spray head are formed on the cup shaped inner circumferential wall of the spray head (see Japanese Examined Patent Publication No. 30-7088). Another rotary type electrostatic spray painting device has been disclosed in which the knife edge shaped tip of the spray head is corrugated (see Japanese Examined Patent Publication No. 36-1266). Other rotary type electrostatic spray painting devices also have been disclosed, in which a plurality of paint guide grooves are formed on the inner circumferential wall of the knife edge shaped tip of the spray head (see Japanese Examined Patent Publication No. 55-41825, Japanese Unexamined Utility Model Publication No. 58-174263, U.S. Pat. No. 4,148,932, U.S. Reissue Pat. No. 31,590, and U.S. Pat. No. 4,458,844).
However, when the tip of the spray head is provided with a knife edge shape, the electrical field becomes stronger at the knife edge shaped tip of the spray head and, therefore, an extremely large dark current flows between the knife edge shaped tip of the spray head and the surface to be painted. This extremely large dark current causes painting defects such as "flower spots" or "cratering" to appear on the painted surface.
To prevent this strengthening of the electrical field at the tip of the spray head, a rotary type electrostatic spray painting device has been disclosed in which the tip of the spray head is provided with an annular flat face perpendicular to the rotating axis of the spray head, and a plurality of paint guide grooves are formed on only the inner peripheral portion of the annular flat face (see Japanese Examined Patent Publication No. 60-57908).
In addition, to prevent an accumulation of paint dregs on the cup-shaped inner circumferential wall of the spray head and in the paint guide grooves, a thin layer of a plastic is formed on the cup-shaped inner circumferential wall of the spray head and on the surfaces of the paint guide grooves (see Japanese Unexamined Utility Model Publication No. 58-190456).
Furthermore, to prevent wear of the cup-shaped inner circumferential wall of the spray head and the surface of the paint guide grooves caused by the inorganic pigments contained in a paint, the present applicant has proposed a rotary type electrostatic spray painting device in a copending Japanese Utility Model Application No. 61-194126. In this device, a hardened layer formed by an anode oxidizing treatment is applied on the cup shaped inner circumferential wall of the spray head and on the surface of the paint guide grooves.
Note, as mentioned above, the atomization of the paint is greatly influenced by tip of the spray head, and thus the tip of the spray head and the paint guide grooves are normally formed by a machining operation. Consequently, in a conventional spray head, the paint guide grooves are normally formed so that they open onto the tip face of the spray head, to facilitate the machining operation. However, if the paint guide grooves are open at the tip face of the spray head, the chances that the openings of the paint guide grooves will come into contact with other articles are increased, particularly when the spray head is removed. This could result in a problem of a deformation of the openings of the paint guide grooves, preventing a proper flow of paint.